/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.hbase.zookeeper;

import java.util.concurrent.CountDownLatch;

/**
 * Placeholder of an instance which will be accessed by other threads
 * but is not yet created. Thread safe.
 */
class InstancePending<T> {

    // Based on a subtle part of the Java Language Specification,
    // in order to avoid a slight overhead of synchronization for each access.
    private final CountDownLatch pendingLatch = new CountDownLatch(1);

    /**
     * Piggybacking on {@code pendingLatch}.
     */
    private InstanceHolder<T> instanceHolder;

    private static class InstanceHolder<T> {
        // The JLS ensures the visibility of a final field and its contents
        // unless they are exposed to another thread while the construction.
        final T instance;

        InstanceHolder(T instance) {
            this.instance = instance;
        }
    }

    /**
     * Returns the instance given by the method {@link #prepare}.
     * This is an uninterruptible blocking method
     * and the interruption flag will be set just before returning if any.
     */
    T get() {
        InstanceHolder<T> instanceHolder;
        boolean interrupted = false;

        while((instanceHolder = this.instanceHolder) == null) {
            try {
                pendingLatch.await();
            } catch(InterruptedException e) {
                interrupted = true;
            }
        }

        if(interrupted) {
            Thread.currentThread().interrupt();
        }
        return instanceHolder.instance;
    }

    /**
     * Associates the given instance for the method {@link #get}.
     * This method should be called once, and {@code instance} should be non-null.
     * This method is expected to call as soon as possible
     * because the method {@code get} is uninterruptibly blocked until this method is called.
     */
    void prepare(T instance) {
        assert instance != null;
        instanceHolder = new InstanceHolder<>(instance);
        pendingLatch.countDown();
    }
}
